Video amplifier



May 18, 1948 E. D. GOODALE ET AL 2,441,880

' VIDEO AMPLIFIER Filed Nov., 29, 1944 IN1/mfom VERNON J.- DUKE a, AELA/"5R DUDLEY GooDALE ATTORNEY Patented May 18, 1948 ."ElinerfDutlleyfGodaie', Bayside Aivmgn'ni; mnuke; mmlwilleE or'sito Radio Gorp'oration v"iviiis invention reiaiesnarticiiisiiy toteievision cuits are utilizedto restore `tlie"D. C. vciiip'ofent of the video or r'nagegs'ignalswhere thelD. C. componentY hasibeenfrernoved'ie t`o .amplification of '.thesigri'als in 'amplifiers 'f the AJ C. type. Further the invention is .particularly lconcerned. With television apparatus and circuits bfia. "forfn sh thattheinsertionfof the direct 'curre't'conidonent` dfthesignalfis ma'deI in such a.riianrier"tlat it .anpears inthe signalapplied-to thejrld orinpticirct lof the .p ow'er `amp1ien-tbe b'f 'ajtr'ansminer. e

'Various Ways andi means Vhave Lallreadyibenjdescribed in fthe `Iiatelted and;pul'ilsled vat Vi'ilhith provide 'for leihseltlhg-ithe direct 'cllfhtfc'lponent .of a Videosignal at .both 'the ftrans'riittr andlat the .irecever endoftlvision/systems. These'D. "0. reinserting vf'z'irciiits lsu'a'lly comprise a diode'tbe which is/.conneetediacros's"theinpt' Accordingly, as one. `ofthe aimslandobjectiires ofthe. present inyention, recourse :is hadtda sys'- DEC. Video signal component. "'Ilieinvition Yaiiipiiiier,miljeteniiiiigtiwarystabilized loadfresstor f the *cathbdeiffollower "driver"-ti.ibe andtherbyifaryngfthe'bias iestablishedifor normali'sperationonithe;power amplil'er.

`It 7'may'. 'i'f desir'ed, "also "be "considered 2 that no viding increased stability of operation of aiivgr :meer we;

iistiiiirstiieir obie'cts o f the iriveiitioiilarethoseef verciineuirouehfthe useiofireia'tively s'im'pre circuits ensei-#mere known itierects of lp'rio' ar-"St systm's'atliforestablishing afrelay; system which and control electrode of a power ampller stage (not shown) as is exemplied by Fig. 1.", The input signals which are impressed upon the'terminals may be considered as being video sig-v nals from which the D. C.s component vhas been removed. These signals are applied to the grid or control electrode l of an amplier tube |1 which may be regarded as having, its cathode .I9

connected to ground at 2| and its anode 23 connected to a source of positive voltage (marked +B) connected at terminal 25 with the load resistor 21-being intermediate the anode 23 and the terminal 25. .Coupling of the output/signal from the ampliiier |1 to the Ycontrol electrode 29 and thev ampliiier 3| is through'the coupling condenser 33.V The direct current reinserting circuit, which is ofthe general nature set'forth and,V claimed in U. S. Patent No. 2,240,281, granted to R. C. Ballard onrApril 29, 1941, comprises essentially a rectiiier tube 35, such as a diode, which has its cathode 31 connected to the lower end of resistor 39 by way of the condenser 4| and the anode element 43 thereof connects to ground Vor atleast to a negative terminal point 45,' as indicated. The complete dioder35 issh'unted' by a resistor 41. v

A s is well known in the television art, the D. C. potential which is a measure of the background brilliance of the television subject which is to be transmitted, appears at'4 the cathode element 31 ofthe diode 35andthis voltage is connected to 'the grid or controll electrode 29 in tube 3| by' way of the resistor element 49. Where desired, a variation oi the said bias, which is applied tothe tube 3|, may be eiected inv known vmanner byY adjustingthe potential of the cathoderelative to' tliegrd'.' 7, Y. `Y Operating voltages for the tube 3| vare applied from a terminal 5| (marked also l -l-B)v fromV which `-voltages. are applied to they anode' 53 and the screen electrode 55. Accordingly, there appears at the output of the tube 3| across the .cathode-loadresistor 51, a voltage `which Vrepresents the signal input, conventionally shown by the `curvesfinmediately above tube |-1, with the D.C. component restored. Thisoutput voltage' is then passed along the conductor 59 vto the output terminal |3 for the purposeof modulating the grid of the power amplifier.- By an adjustmentor the Vbias applied at the vterminal point @l5-which serves as the D.C. bias. in the system, and taking into account the Vvoltage A,drop occurring in thisresistor vI1 due tothe flowof current in the cathode follower tubeL3I, it is apparent thatvarlous waysto adjust the-normalbias applied tothe grid of the power amplier, (not shown) connected to terminal .|3--may be estab-V lished.l Y

In the modification of the invention shown by Fig. l2, the circuit comprises essentially the ele-- ments already described `in connection with Fig. l and for like parts like numerals have been ap.A plied. However, for the purpose oifprovding compensation'for any-tendency of the bia'son the. power ampliertube 6| to vary recourse. is had to a compensatingtube 63 which is-connected in the circuit in a manner later to be described. It

can be appreciated that with the D.C. com-f ponent of the modulating signal inserted at ,the grid 29 of the cathode follower tube 3| by means of the diode 35 due to the follower characteristics of the tube 3|, the voltage, which appears in the conductor 59 across the resistor 51 connected to the cathode 56 of tube 3|, isV of like polarity to the signal applied to the input grid 29. The signal infconductor 59 also has the proper D.C. com- 10 ponent s o that this proper D.C. component will be inserted into the nal transmitted or relayed -sign-aLproviding no appreciable change on the bias? of thefpow'eramplier tube 6| occurs.

It is, anwellfknown fact that in transmitter operations',ia tendency always exists for the power ,amplientuba when grid modulated, to draw a lrelatively heavy grid current, and to this extent, when signal voltages, such as those represented adjacent the c-onductor 59, for instance, are applied to the modulating grid 65 of the power amplier tube which has its cathode 51 connected to ground 2|, there will be a tendency for grid current to flow within the tube. This grid current iiow will then ilow through the cathode resistor 51 of the cathode Vfollower stage in the absence of any compensation and, unless .the current of the cathode Vfollower is very large in comparison'tothe grid current 0f the power amplier, a change in bias on the power amplier tube will Voccur due tovoltage .drop in the cathode resistor 51. However, due to the fact that the power amplifier tends to draw under normal conditions an appreciable grid current, provisions are made by this invention for a compensating tube, 63 to nullify the detrimental effects resulting from this condition.

Itis apparent that this effect of grid current flow inthe power amplierwill not be as pronounced with a low value for the cathode resistor 51 but it is extremely. ineii'icient operation of the cathode follower' stage to use a small value of load resistance. i It is. known that ifrthe resistor 51 is of an Aappreciable value, such for instance of the s order of"1,Q00 to 1,500, ohms (D.C. value) and a plate currentflow vthrough tube 3| is of the order of milliamperes a higher efliciency of the cathode follower stagewill result than if a low output resistor is used. However, without compensation v it can be seen that grid current flow from the power :amplifier which is Vdirectly connected to conductor 59 will ilow Vthrough theload resistor `51 and produce therein a substantial voltage drop to react at the power ampliergrid tochange its i bias andinake lthe operation unsatisfactory.

To counteract the effect ofthis voltage varia` tion'thls invention provides a compensating'tube 63 which has; its grid or controlV electrode 1| connected by Way V0f conductor 13 nand the resistor15,.with1capacity11 connected Ato ground,

` as shown, to the point 8| whereat the corrected D.C.",voltageappearaV s0 rthat' in naccordance with the potential appearing Yuponythe Vgrid 1| of the. compensating tube 63Vrelativeto the cathodeBS thereof,` a tendency exists for current to Yflow .within the'tube 6,3 VinY a direction .between the tube cathode and the plate or anode 19, which latter electrode connects at terminal point 8l jrto Yasourcef positive operating potential. The bias` onthe tube63 will Yautomatically befadjusted in accordance .with the time constant of VVthe circuit including resistor 1,5 and capacity ,11. jUnder r,these conditions, it will be appreciatedthatthe use of the Vcompensating tube 931s such that Vitprevents any grid current tending to flow through the power amplifier froml nowing' through `the cathode` load resistorVv 51;

y wherebyachangein bias on power` ampliiier grid $`v would-beoccasioned Vso theL currentfi-'o'wjv the tubeM ed 'thus Ycan bomadeto balance and! ductancein tube 63`itispossible to create the eiectsof over D. C.k ins'ertion at the power amplifier." VSiichover' D. C.' Yir'niertionmay beuti-f lizedH to overcome the detriiner'it'al--v effects of cathode Ibias on the cathode ofltlieV power-arnplizlertube, where desired.

Various-mod-ihcations ofthe sys-'tem .her

forth vand claimedf may? ofcourse be relied upon andisbelievedfthat any and all of these modifi-'fl cations may readily be used so long as they fall fairly' within the -spirit'and scope offwhatis hereinaffter'claime'd. Y

Wef'claim: Y 1'. a` video f circiiitfwherei'n video signals are tobe' transferred from a signal inputan outputstabilizedfmodulating load circuit which includes a power ampl-iier" tube which tends to draw gridcurrent when signal modulated the-'combination which comprises a coupling tube connected intermediate the point of Video signal input andthe grid input circuit of the power amplifier' tube', a D. C. re-

insertingjunitffconnected:torrespondto the input video signals and to supply therefrom the D; C. componentat the input ofthe Vcoupling tube, a cathode follower'connection"from the coupling tube cathode to the grid inputfff the poweranif plier tubeztoasupply. thereto lfoutpu't signals from the coupling tube in which the D. C. component has been restored and a compensating and voltage regulator tube connected in parallel with the input circuit of the power amplifier tube to maintain a predetermined bias upon the power amplier.

2. In a television circuit wherein video signals are to be transferred from a signal input point to a load circuit including a power amplifier tube which tends to draw grid current when signal modulated the combination which comprises a coupling tube connected intermediate the point of video signal input and the grid input circuit of the power amplifier tube, means for re-inserting the D. C. component of the impressed Video signals at the input of the coupling tube, a cathode resistor connected to the coupling tube cathode across which output signals from the from a predetermined point on the cathode resistor to the grid input of the power amplifier tube to supply thereto output signals from the coupling tube in which the D. C. component has been restored and a compensating thermionic tube connected in parallel with the cathode resistor to compensate for any tendency of grid current flow in the power amplier tube tending to produce a voltage drop in the said cathode resistor which would alter the bias of the power amplifier tube.

3. The circuit claimed in claim 1 comprising in addition, means to control the current flow through the voltage regulator tube in accordance with the control of the re-inserted D. C. signal component.

4. In a television circuit wherein video signals are to be transferred from an input to a load gioioso I' circuit which includes a power amplifier ltubo` which tends to draw grid currentl when signal. modulated! the combinationwhicli comprises' a:

- cathode across which output sign-als.' 'from the:-

couplingtube may bend-erived., a fdi-re'ct'fconnection `from a predeterminedppo" iti-oni thec'athode resistor t'o' the gri-'d input 'of the. power amplier tube'v to' supply thereto routput signals*V 4from the coupling'tube which the D'. C; componentifhas been restored-,- a compensating. tlliermionic 'tube'. connected in parallel with the. cathode resistor to compensatefor any tender-i'cy'V ofi grid current nowtnef power ampliner tube tending ,to .produce a Voltageidropiin th'efs'aid'cathode resistor which would"v alter the bias 'of 'the power amplifier-tube, and means to'. control.' the current flowing in the' compensating ti-ib'e in accorde,- ancelwith the re-ins'e'rtedD; component :of the impressed vsig-nal. i i

5'. The combination claimed claim i comev prising, iin addition; a time constant'circuitiior maintaining the` 'effect offthe re-insertedl. component controlling 'the current; flow through the compensating 'tube for'pred'eternnnedtime periods. i

of tel-evisionfci-rcuit wherein: video signals are-"tope transterre'difroman input circuitto ak load circuit medicatedthe: combination; comprises; a

video si-'gnal input' and thek `'grid hiput cireu-'rtl'pf :the power amplifier tube, means for re-inserting the D. C. component of the impressed video signals at the input of the coupling tube, a cathode resistor connected to the cathode of the coupling tube, a direct connection from a predetermined` which tends to draw grid current when signal' modulated the combination which comprises a coupling tube having an input circuit connected to receive the video signal input and a cathode follower output circuit directly connected to supply output signals to the grid input circuit of the power amplifier tube, means for re-inserting the D. C. component of the impressed video signals in the input of the coupling tube so that the output signal includes the D. C. component, and a voltage regulator tube connected in parallel with the cathode follower output to maintain the grid circuit impedance of the power amplifier tube substantially constant.

8. The circuit claimed in claim 7 comprising, in addition, means to control the current flow in the voltage regulator tube in proportion to the D. C. component signal restored 'and supplied to the coupling tube.

9. The circuit claimed in claim 7 comprising, in

" cluding :a power.' tube draw' gri'd'currentwhenx'signal .g 'tube connectedlinztermediatefthepoint ci addition, means to control the current flow through the voltage regulator tube from the re'- stored D. C. component signal and a time constant circuit to maintain the control voltage eiective in the Voltage regulator tube for predetermined time periods.

10. In a television circuit wherein video Vsignals are to be transferred from an input to a load circuit including a power amplifier tube which tends to draw grid current when signal modulated the combination which comprises a coupling tube connected intermediate the point of Video signal input and the grid input circuit of the power amplifier tube, signal rectifier means for reinserting the D. C. component 0f the impressed video signals at the input of the coupling tube, a, cathode resistor connected to the coupling tube cathode so that output signals may be derived therefrom in which the direct current component has been added, a direct connection from a predetermined point on the cathode resistor to the grid input of the power ampliiier tube to supply thereto the output signals from the coupling tube, a compensating tube connected in parallel with the signal supply to the power ampliiier tube grid input to maintain the grid circuit impedance of the power amplifier tube substantial-ly constant, and a connection to supply a signal representative of the D. C. component of the input signal to the compensating tube to control the current iiow therethrough.

11. In a television circuit wherein video signals in which the D.C. component has been restored are to be transferred to a load circuit including a. power ampliiier tube which tends to draw grid current when signal modulated the combination which comprises a coupling tube connected intermediate a point of Video signal input and the grid input circuit of the power amplifier tube for transferring the'video signals with the re-inserted D.C. component from the signal input circuit to the power amplier tube, a cathode resistor connected to the coupling tube cathode across which resistor output signals from the coupling tube are derived, a galvanic connection from a selected point on the said cathode resistor to the grid input of the power amplier tube to supply output signals in which the D.C. component has been restored from the coupling tube to the power ampliiier tube, a voltage regulator unit connected in parallel with the signal supply to the power amplifier tube grid input to maintain the grid circuit impedance of the power amplifier tube substantially constant. Y

12. The circuit claimed in claim 11 comprising, in addition, means to control the current flow in the Voltage regulator tube in Yproportion to the D.C. component signal restored and supplied to the coupling tube. Y

13. The circuit claimedin claim 11 comprising, in addition, means to control the current iiow vthrough the voltage regulator tube from'the restored D.C. component signal and a time conlstant circuit to maintain the control voltage effective in the voltage regulator tube for predetermined time periods.

ELMER DUDLEY GOODALE. VERNON J. DUKE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED v STATES PATENTS Number Name Date 2,240,281 BallardV Apr. 29, 1941 

